The present invention relates to a novel coating composition for forming a titanium oxide film, process for forming a titanium oxide film, and use of the coating composition.
Metal substrates such as steel sheets, aluminum and aluminum alloys are usually subjected to various types of surface treatment (undercoating) to improve the corrosion resistance, coating adhesion, processability, etc.
In recent years, surface-treated steel sheets are required to have higher corrosion resistance, and therefore zinc-based metal plated steel sheets are frequently used as metal substrates replacing cold rolled steel sheets.
Conventionally, chromate treatment or phosphate treatment is employed for surface treatment of zinc-based metal plated steel sheets.
Chromate treatment have problems with inherent toxicity of chromium compounds. In particular, hexavalent chromium compounds are extremely harmful substances designated human carcinogens by IARC (International Agency for Research on Cancer Review) and many other public organizations. Specifically stated, chromate treatment has problems with chromate fumes produced during the treatment process, extremely high cost required for waste water disposal equipment, chromic acid dissolved out from chromate treatment coats, and the like.
Phosphate treatment using zinc phosphate, iron phosphate or like phosphate is usually followed by chromate post-treatment, and thus involves the problems with toxicity of chromium compounds. Moreover, phosphate treatment has problems with disposal of waster water which contains reaction accelerators, metal ions and the like owing to phosphate treatment agents, and sludge disposal necessitated by metal ions dissolved out from treated metals.
As another post-treating agent for phosphate treatment coats than chromate, Japanese Examined Patent Publication No. 1995-42423 proposes a post-treating composition containing metal ions such as titanium ions or zirconium ions and polyalkenyl phenol. However, when the proposed post-treating composition is used for post-treatment of a phosphate treatment coat on a zinc-based metal plated steel sheet, the composition does not sufficiently inhibit white rusting and thus hardly improves the corrosion resistance.
Japanese Unexamined Patent Publications No. 1983-224174, No. 1985-50179 and No. 1985-50180 disclose coated steel sheets comprising a zinc-based metal plated steel sheet substrate, a chromate coat formed on the substrate and an organic silicate coat formed on the chromate coat. The disclosed coated steel sheets are excellent in corrosion resistance and processability, but have the problems with toxicity of chromium compounds owing to the chromate coat. Without the chromate coat, the coated steel sheets have insufficient corrosion resistance.
Aluminum or aluminum alloy substrates are also subjected to various types of surface treatment (undercoating) in many cases, to improve the corrosion resistance, coating adhesion and other properties.
Generally, fins in heat exchangers for air conditioners are made of aluminum or aluminum alloy substrates which are light in weight and excellent in processability and thermal conductivity. In air conditioner heat exchangers, water condenses into droplets and forms water bridges between the fins during cooling operation. The bridges narrow the passageway for air and increase the resistance to air passage, thus causing problems such as power loss, noise, water splashing, etc.
To solve these problems, surfaces of the aluminum or aluminum alloy fins are subjected to boehmite treatment, water glass coating, aqueous polymer coating or like hydrophilizing treatment for preventing bridge formation. However, in a highly corrosive environment, the hydrophilized aluminum or aluminum alloy fins are corroded within a few months or so, partly because of the hydrophilicity of the treatment coat.
To prevent corrosion of the fins, chromate treatment is often employed for undercoating of aluminum or aluminum alloy substrates, since chromate treatment has the advantages of providing good corrosion resistance with low cost. However, chromate treatment is accompanied by the problems with toxicity of chromium compounds as described above.
As chromate-free undercoating materials and undercoating processes, Japanese Unexamined Patent Publication No. 1979-24232 discloses treatment of an aluminum surface with an acid solution comprising a titanium salt, hydrogen peroxide and condensed phosphoric acid; Japanese Unexamined Patent Publication No. 1979-160527 discloses treatment of an aluminum surface with an aqueous alkaline solution containing titanium ions and a complexing agent, followed by water washing and treatment with an aqueous solution of an acid such as phosphoric acid; Japanese Unexamined Patent Publication No. 1997-20984 discloses an aluminum surface treating agent comprising phosphoric acid ions, a titanium compound and a fluoride; and Japanese Unexamined Patent Publication No. 1997-143752 discloses an aluminum-based metal surface treating agent comprising condensed phosphate, a titanium salt, a fluoride and phosphite.
However, these undercoating materials and processes utilizing titanium compounds have the problems such as insufficient stability of the undercoating materials, lower corrosion resistance of the coat than a chromate treatment coat, insufficient hydrophilicity and insufficient durability of the coat.
In view of the above state of the art, there are demands for an inorganic film-forming material which is useful as an undercoating material for metal substrates such as steel sheets, aluminum, aluminum alloys or the like, or as a post-treating agent for phosphate undercoats, and which is capable of forming a film excellent in corrosion resistance and other properties without causing toxicity problems.
An object of the present invention is to provide a novel coating composition and process for forming a titanium oxide film, which are capable of forming, on a metal substrate, an undercoat excellent in corrosion resistance, processability and coating adhesion.
Another object of the invention is to provide an undercoating material for a metal substrate, which is capable of forming a film excellent in corrosion resistance, processability and coating adhesion.
A further object of the invention is to provide a post-treating agent for phosphate undercoats on metal substrates, which is capable of forming a film excellent in corrosion resistance, processability and coating adhesion.
Other objects and features of the present invention will become apparent from the following description.
The invention provides the following novel coating compositions for forming a titanium oxide film, processes for forming a titanium oxide film, and use of the coating compositions:
1. A coating composition for forming a titanium oxide film, comprising (A) a titanium-containing aqueous liquid obtained by mixing at least one titanium compound selected from the group consisting of hydrolyzable titanium compounds, low condensates of hydrolyzable titanium compounds, titanium hydroxide and low condensates of titanium hydroxide with aqueous hydrogen peroxide, and (B) at least one halide selected from the group consisting of titanium halides, titanium halide salts, zirconium halides, zirconium halide salts, silicon halides and silicon halide salts.
2. A coating composition according to item 1, wherein the titanium-containing aqueous liquid (A) is an aqueous peroxo titanic acid solution obtained by mixing a hydrolyzable titanium compound and/or its low condensate with aqueous hydrogen peroxide.
3. A coating composition according to item 2, wherein the hydrolyzable titanium compound is a tetraalkoxytitanium represented by the formula
Ti(OR)4xe2x80x83xe2x80x83(1) 
wherein Rs may be the same or different and each represent C1 to C5 alkyl.
4. A coating composition according to item 2, wherein the low condensate of a hydrolyzable titanium compound is a compound having a condensation degree of 2 to 30 and obtained by self-condensation of tetraalkoxytitanium(s) represented by the formula
xe2x80x83Ti(OR)4xe2x80x83xe2x80x83(1)
wherein Rs may be the same or different and each represent C1 to C5 alkyl.
5. A coating composition according to item 2, wherein the proportion of the aqueous hydrogen peroxide is 0.1 to 100 parts by weight calculated as hydrogen peroxide, per 10 parts by weight of the hydrolyzable titanium compound and/or its low condensate.
6. A coating composition according to item 2, wherein the titanium-containing aqueous liquid (A) is an aqueous peroxo titanic acid solution obtained by mixing a hydrolyzable titanium compound and/or its low condensate with aqueous hydrogen peroxide in the presence of a titanium oxide sol.
7. A coating composition according to item 6, wherein the titanium oxide sol is an aqueous dispersion of anatase titanium oxide.
8. A coating composition according to item 6, wherein the proportion of the titanium oxide sol is 0.01 to 10 parts by weight as solids, per 1 part by weight of the hydrolyzable titanium compound and/or its low condensate.
9. A coating composition according to item 1, wherein the halide (B) is at least one member selected from the group consisting of titanium hydrofluoric acid, titanium potassium fluoride, titanium ammonium fluoride, zirconium hydrofluoric acid, zirconium ammonium fluoride, zirconium potassium fluoride, hydrosilicofluoric acid, sodium silicofluoride, ammonium silicofluoride and potassium silicofluoride.
10. A coating composition according to item 1, wherein the proportion of the halide (B) is 10 to 300 parts by weight per 100 parts by weight of the solids in the titanium-containing aqueous liquid (A).
11. A coating composition according to item 1, which has a pH of 2 to 10.
12. A process for forming a titanium oxide film, comprising applying a coating composition according to item 1 to a metal substrate and drying the resulting coat.
13. An undercoating material for a metal substrate, which is a coating composition according to item 1.
14. An undercoating material according to item 13, wherein the-metal substrate is a steel sheet.
15. A coated steel sheet comprising a film of an undercoating material according to item 14 formed on a steel sheet surface.
16. A coated steel sheet according to item 15, wherein the film is 0.05 to 10 xcexcm thick.
17. A post-treating agent for phosphate undercoats on metal substrates, which is a coating composition according to item 1.
18. A post-treating agent according to item 17, wherein the metal substrate is a steel sheet.
19. A coated substrate comprising a film of a post-treating agent according to item 17 formed on a surface of a phosphate undercoat on a metal substrate.
20. A coated substrate according to item 19, wherein the weight of the film of the post-treating agent is 0.05 to 3.0 g/m2.
21. An undercoating material according to item 13, wherein the metal substrate is made of aluminum or aluminum alloy.
22. A coated substrate comprising a film of an undercoating material according to item 21 formed on a surface of an aluminum substrate or an aluminum alloy substrate.
23. A coated substrate according to item 22, wherein the film is 0.001 to 10 xcexcm thick.
24. A heat exchanger fin comprising an aluminum substrate or an aluminum alloy substrate, a film of an undercoating material according to item 21 formed on the substrate, and a hydrophilizing coat formed on the film.
25. A fin according to item 24, wherein the film of the undercoating material is 0.001 to 10 xcexcm thick, and the hydrophilizing coat is 0.3 to 5 xcexcm thick.
The present inventors carried out extensive research to achieve the above objects, and found that the above specified coating compositions are capable of forming, on a metal substrate, a film excellent in corrosion resistance, processability and coating adhesion and thus suitable as an undercoat. They further found that the coating compositions are suitable as post-treating agents for phosphate undercoats on metal substrates.
The present invention has been accomplished based on these novel findings.
Coating Composition for Forming a Titanium Oxide Film
The coating composition for forming a titanium oxide film according to the present invention is an aqueous coating composition comprising (A) a titanium-containing aqueous liquid and (B) a halide, as described above.
The aqueous liquid component (A) for use in the composition of the invention can be suitably selected from known titanium-containing aqueous liquids obtainable by mixing at least one titanium compound selected from the group consisting of hydrolyzable titanium compounds, low condensates of hydrolyzable titanium compounds, titanium hydroxide and low condensates of titanium hydroxide, with aqueous hydrogen peroxide.
The hydrolyzable titanium compounds are titanium compounds each containing a hydrolyzable group or groups bonded directly to a titanium atom. The compounds produce titanium hydroxide when reacted with water, water vapor or the like. In the hydrolyzable titanium compounds, the groups bonded to the titanium atom may be all hydrolyzable groups, or part of the groups may be previously hydrolyzed to hydroxyl groups.
The hydrolyzable groups may be any groups capable of producing hydroxyl groups by reaction with water. Examples of such groups include lower alkoxyl and groups forming salts with titanium atoms. Examples of the groups forming salts with titanium atoms include halogen atoms (e.g., chlorine atoms), hydrogen atoms and sulfuric acid ions.
Examples of hydrolyzable titanium compounds containing lower alkoxyl groups as hydrolyzable groups include tetraalkoxytitaniums.
Typical examples of hydrolyzable titanium compounds containing, as hydrolyzable groups, groups forming salts with titanium include titanium chloride and titanium sulfate.
The low condensates of hydrolyzable titanium compounds are products of low self-condensation of the hydrolyzable titanium compounds. In the low condensates, the groups bonded to the titanium atom may be all hydrolyzable groups, or part of the groups may be previously hydrolyzed to hydroxyl groups.
Examples of low condensates of titanium hydroxide include orthotitanic acid (titanium hydroxide gel) obtained by reaction of an aqueous solution of titanium chloride, titanium sulfate or the like with an aqueous solution of an alkali such as ammonia or caustic soda.
The low condensates of hydrolyzable titanium compounds or low condensates of titanium hydroxide have a condensation degree of 2 to 30, preferably 2 to 10.
The aqueous liquid (A) may be a known titanium-containing aqueous liquid obtained by reaction of the above titanium compound with aqueous hydrogen peroxide. Specific examples of such aqueous liquids include the following:
(1) Aqueous peroxo titanic acid solutions described in Japanese Unexamined Patent Publications No. 1988-35419 and No. 1989-224220, obtained by adding aqueous hydrogen peroxide to a gel or sol of hydrous titanium oxide;
(2) Yellow, transparent, viscous aqueous liquids for forming titanium oxide films, described in Japanese Unexamined Patent Publications No. 1997-71418 and No. 1998-67516, obtained by: reacting an aqueous solution of titanium chloride, titanium sulfate or the like with an aqueous solution of an alkali such as ammonia or caustic soda to precipitate a titanium hydroxide gel called orthotitanic acid; isolating the titanium hydroxide gel by decantation; washing the isolated gel; and adding aqueous hydrogen peroxide to the gel;
(3) Titanium oxide film-forming aqueous liquids described in Japanese Unexamined Patent Publications No. 2000-247638 and 2000-247639, obtained by: adding aqueous hydrogen peroxide to an aqueous solution of an inorganic titanium compound such as titanium chloride or titanium sulfate to prepare a peroxo titanium hydrate; adding a basic substance to the peroxo titanium hydrate; allowing to stand or heating the resulting solution to precipitate a peroxo titanium hydrate polymer; removing dissolved components other than water; and allowing hydrogen peroxide to act.
Preferably, the titanium-containing aqueous liquid (A) is an aqueous peroxo titanic acid solution (A1) obtained by mixing a hydrolyzable titanium compound and/or its low condensate with aqueous hydrogen peroxide.
A particularly preferred example of the titanium compounds is a tetraalkoxytitanium represented by the formula
Ti(OR)4xe2x80x83xe2x80x83(1) 
wherein Rs may be the same or different and each represent C1 to C5 alkyl. Examples of C1 to C5 alkyl represented by R include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl and tert-butyl.
The low condensates of titanium compounds are preferably self-condensates of the compounds of the formula (1) having a condensation degree of 2 to 30, preferably 2 to 10.
The proportion of the aqueous hydrogen peroxide is preferably 0.1 to 100 parts by weight, particularly 1 to 20 parts by weight, calculated as hydrogen peroxide, per 10 parts of the hydrolyzable titanium compound of the formula (1) and/or its low condensate (hereinafter the compound and/or its low condensate is referred to simply as xe2x80x9chydrolyzable titanium compound (I)xe2x80x9d). Less than 0.1 part by weight of aqueous hydrogen peroxide (calculated as hydrogen peroxide) will result in insufficient formation of peroxo titanic acid, producing opaque precipitates. On the other hand, if more than 100 parts by weight (calculated as hydrogen peroxide) of aqueous hydrogen peroxide is used, it is likely that part of hydrogen peroxide remains unreacted and emits hazardous active oxygen during storage.
The hydrogen peroxide concentration in the aqueous hydrogen peroxide is not limited, but is preferably 3 to 40 wt. %, considering ease of handling.
The aqueous peroxo titanic acid solution can be prepared usually by mixing the hydrolyzable titanium compound (I) with aqueous hydrogen peroxide with stirring at about 1 to 70xc2x0 C. for about 10 minutes to 20 hours. If necessary, methanol, ethanol, n-propanol, isopropanol, ethylene glycol monobutyl ether, propylene glycol monomethyl ether or like water-soluble solvent may be also mixed.
Presumably, the aqueous peroxo titanic acid solution (A1) is obtained through the following mechanism: When the hydrolyzable titanium compound (I) is mixed with aqueous hydrogen peroxide, the compound is hydrolyzed with water and formed into a hydroxyl-containing titanium compound. Immediately thereafter, hydrogen peroxide is coordinated to the hydroxyl-containing titanium compound to thereby form peroxo titanic acid. The aqueous peroxo titanic acid solution is highly stable at room temperature and durable for long-term storage.
Also preferred is an aqueous peroxo titanic acid solution (A2) obtained by mixing the hydrolyzable titanium compound (I) with aqueous hydrogen peroxide in the presence of a titanium oxide sol, since this solution has improved storage stability and is capable of forming a titanium oxide film improved in corrosion resistance and other properties. The reason for the improvements is presumed as follows: During preparation of the aqueous solution, the hydrolyzable titanium compound (I) is adsorbed on the titanium oxide sol particles and chemically bonded by condensation to hydroxyl groups generated on the particle surface. Further, the hydrolyzable titanium compound undergoes self-condensation and is converted into a high molecular compound. The high molecular compound is mixed with aqueous hydrogen peroxide, thereby giving a stable aqueous peroxo titanic acid solution remarkably free of gelation and thickening during storage.
The titanium oxide sol comprises amorphous titanium oxide particles or anatase titanium oxide particles dispersed in water. As the titanium oxide sol, an aqueous dispersion of anatase titanium oxide is preferred from the viewpoint of corrosion resistance. The titanium oxide sol may contain, in addition to water, an aqueous organic solvent such as an alcohol solvent or an alcohol ether solvent.
The titanium oxide sol may be known one, such as a dispersion of amorphous titanium oxide particles obtained by dispersing titanium oxide agglomerates in water, or a dispersion in water of anatase titanium oxide particles obtained by calcining titanium oxide agglomerates. Amorphous titanium oxide can be converted into anatase titanium oxide by calcination at a temperature not lower than the anatase crystallization temperature, usually at a temperature not lower than 200xc2x0 C. Examples of titanium oxide agglomerates include (1) agglomerates obtained by hydrolysis of an inorganic titanium compound such as titanium sulfate or titanyl sulfate, (2) agglomerates obtained by hydrolysis of an organic titanium compound such as titanium alkoxide, (3) agglomerates obtained by hydrolysis or neutralization of a solution of titanium halide such as titanium tetrachloride.
Commercially available titanium oxide sols include, for example, xe2x80x9cTKS-201xe2x80x9d (tradename, an aqueous sol of anatase titanium oxide particles with an average particle size 6 nm manufactured by TEICA Corp.), xe2x80x9cTKS-203xe2x80x9d (tradename, an aqueous sol of anatase titanium oxide particles with an average particle size of 6 nm manufactured by TEICA Corp.), xe2x80x9cTA-15xe2x80x9d (tradename, an aqueous sol of anatase titanium oxide particles manufactured by Nissan Chemical Industries, Ltd.) and xe2x80x9cSTS-11xe2x80x9d (tradename, an aqueous sol of anatase titanium oxide particles manufactured by Ishihara Sangyo Kaisha, Ltd.).
The amount of the titanium oxide sol used when mixing the hydrolyzable titanium compound (I) and aqueous hydrogen peroxide is, as solids, usually 0.01 to 10 parts by weight, preferably 0.1 to 8 parts by weight, per 1 part by weight of the hydrolyzable titanium compound (I). Less than 0.01 part by weight of the titanium oxide sol fails to achieve the effect of adding a titanium oxide sol, i.e., improvement of storage stability of the coating composition and corrosion resistance of the titanium oxide film. On the other hand, more than 10 parts by weight of the sol impairs the film-forming properties of the coating composition.
The titanium-containing aqueous liquid (A) may be used in the form of a dispersion of titanium oxide particles with an average particle size not greater than 10 nm. Such a dispersion can be prepared by mixing the hydrolyzable titanium compound (I) with aqueous hydrogen peroxide optionally in the presence of the titanium oxide sol, and then subjecting the resulting aqueous peroxo titanic acid solution to heat treatment or autoclave treatment at a temperature not lower than 80xc2x0 C. The dispersion usually has a translucent appearance.
If the heat treatment or autoclave treatment is carried out at a temperature lower than 80xc2x0 C., the crystallization of titanium oxide does not proceed sufficiently. The titanium oxide particles obtained by heat treatment or autoclave treatment have a particle size not greater than 10 nm, preferably a particle size of 1 nm to 6 nm. If the titanium oxide particles have a particle size greater than 10 nm, the resulting coat composition has such a low film-forming properties that a film with a thickness of 1 xcexcm or greater develops cracks.
When the aqueous solution (A1) is used as the titanium-containing aqueous liquid (A), the solution usually forms an amorphous titanium oxide film containing a slight amount of hydroxyl groups under the above drying conditions. The amorphous titanium oxide film has the advantage of higher gas barrier properties. When the titanium-containing aqueous solution (A2) is used as the aqueous solution (A), the solution usually forms an anatase titanium oxide film containing a slight amount of hydroxyl groups under the above drying conditions.
The component (B) of the composition of the invention is at least one halide selected from the group consisting of titanium halides, titanium halide salts, zirconium halides, zirconium halide salts, silicon halides and silicon halide salts.
Examples of halogens in the component (B) include fluorine, chlorine and iodine. Among them, fluorine is particularly preferred to achieve excellent storage stability, corrosion resistance, moisture resistance, etc.
The halide salts for use as the component (B) are, for example, salts formed with sodium, potassium, lithium, ammonium or the like. Among them, potassium and sodium are preferred.
Preferred examples of the halide (B) include titanium hydrofluoric acid or like titanium halides; titanium potassium fluoride, titanium ammonium fluoride and like titanium halide salts; zirconium hydrofluoric acid and like zirconium halides; zirconium ammonium fluoride, zirconium potassium fluoride and like zirconium halide salts; hydrosilicofluoric acid and like silicon halides; and sodium silicofluoride, ammonium silicofluoride, potassium silicofluoride and like silicon halide salts.
The proportion of the halide (B) in the coating composition of the invention is 10 to 300 parts by weight, preferably 20 to 150 parts by weight, per 100 parts by weight of the solids in the titanium-containing aqueous liquid (A).
The coating composition of the invention is aqueous and usually has a pH of 2 to 10. Preferably, the coating composition has good storage stability in the acidic region, and has a pH of 2 to 6, in particular 3 to 5.
The coating composition of the invention may optionally contain additives, such as rust preventive agents, titanium oxide sols, titanium oxide powders, extender pigments, rust preventive pigments, coloring pigments and surfactants. Examples of rust preventive agents include tannic acid, phytic acid and benzotriazole. Examples of extender pigments include mica, talc, silica, barium sulfate and clay.
Preferably, the composition of the invention is free from phosphoric acid compounds such as phosphoric acid, polyphosphoric acid, phosphoric acid derivatives, phosphorous acid and hypophosphorous acid, since these compounds reduce the storage stability of the composition for some unknown reason.
The coating composition of the invention has excellent storage stability, and is capable of forming, on a metal substrate, a titanium oxide-containing film excellent in corrosion resistance, processability and coating adhesion. Accordingly, the composition is suitable as an undercoating material for metal substrates and as a post-treating agent for phosphate undercoats.
Process for Forming a Titanium Oxide Film
The process for forming a titanium oxide film according to the invention comprises applying the coating composition of the invention to a metal substrate and drying the resulting coat.
The metal substrate for use in the process of the invention may be any substrate at least having a metal surface. Examples of usable substrates include those having a surface made of iron, aluminum, zinc, copper, tin or an alloy containing any of these metals. Typical examples of such substrates include iron sheets, cold rolled steel sheets, zinc-based metal plated steel sheets, aluminum sheets, aluminum alloy sheets and aluminum plated steel sheets. Examples of zinc-based metal plated steel sheets include hot-dip galvanized steel sheets, electrogalvanized steel sheets, iron-zinc alloy plated steel sheets, nickel-zinc alloy plated steel sheets, and aluminum-zinc alloy plated steel sheets.
The coating composition of the invention can be applied to a metal substrate by any known process, such as dip coating, shower coating, spray coating, roll coating and electrocoating. It is usually preferable that the composition is dried for about 2 seconds to 30 minutes by heating under such conditions that the substrate reaches a maximum temperature of about 60 to 250xc2x0 C.
The dry film of the coating composition is preferably about 0.001 to 10 xcexcm thick.
In this manner, the process of the invention produces, on a metal substrate, a titanium oxide film excellent in corrosion resistance, processability and coating adhesion.
Undercoating Material for Steel Sheets as Metal Substrates
The coating composition of the invention, when used as a undercoating material, is particularly preferred as an undercoating material for steel sheets.
Examples of steel sheet substrates to be coated with the undercoating material of the invention include hot-dip galvanized steel sheets, electrogalvanized steel sheets, iron-zinc alloy plated steel sheets, nickel-zinc alloy plated steel sheets, and aluminum-zinc alloy plated steel sheets. Examples of aluminum-zinc alloy plated steel sheets include those marketed under the tradenames of xe2x80x9cGalvaliumxe2x80x9d and xe2x80x9cGalfanxe2x80x9d. Also usable are zinc-based metal plated steel sheets that have been subjected to chemical conversion treatment such as chromate treatment, zinc phosphorate treatment or composite oxide film treatment. Further, a steel sheet assembly can be employed as a steel sheet substrate.
The coated steel sheet according to the invention comprises a film of the undercoating material of the invention formed on a steel sheet surface. The coated steel sheet can be obtained by applying the undercoating material of the invention to a steel sheet surface and drying the resulting coat.
The undercoating material can be applied to a steel sheet substrate by a known coating process such as dip coating, shower coating, spray coating, roll coating or electrocoating. The undercoating material is preferably dried for about 2 seconds to 30 minutes under such conditions that the substrate reaches a maximum temperature of about 60 to 250xc2x0 C.
The dry film of the undercoating material is preferably 0.05 to 10 xcexcm thick, in particular 0.1 to 3 xcexcm thick. A film of the undercoating material with a thickness less than 0.05 xcexcm is inferior in corrosion resistance, water resistance and other properties. On the other hand, a film of the undercoating material with a thickness greater than 10 xcexcm develops cracks or has reduced processability.
The coated steel sheet of the invention is excellent in corrosion resistance, coating adhesion, processability, fingerprint resistance and other properties, and thus can be used as a rust resistant steel sheet or a lubricated rust resistant steel sheet, without further treatment.
On the coated steel sheet of the invention, an upper coat may be formed as required. The composition for forming the upper coat can be selected from various coating compositions, according to the intended purpose. Examples of coating compositions for forming the upper coat include primer compositions and colored topcoat compositions. It is also possible to apply a primer composition and then further apply a colored topcoat composition on the primer coat.
Post-Treating Agent for Phosphate Undercoats on Metal Substrates
The coating composition of the invention is suitable as a post-treating agent for phosphate undercoats on metal substrates.
The phosphate undercoat to be treated with the post-treating agent of the invention may be formed on any substrate at least having a metal surface. The substrate may be, for example, one having a surface made of iron, aluminum, zinc, copper, tin or an alloy containing any of these metals. Typical examples of such substrates include iron sheets, cold rolled steel sheets, zinc-based metal plated steel sheets, aluminum sheets, aluminum alloy sheets, and aluminum-plated steel sheets. Examples of zinc-based metal plated steel sheets include hot-dip galvanized steel sheets, electrogalvanized steel sheets, iron-zinc alloy plated steel sheets, nickel-zinc alloy plated steel sheets, and aluminum-zinc alloy plated steel sheets.
The phosphate undercoat is formed on the surface of these metal substrates by known phosphate treatment (e.g., iron phosphate treatment or zinc phosphate treatment) carried out by dip coating, roll coating or like process. The weight of the phosphate undercoat on the metal surface is not limited, but is preferably 0.05 to 5 g/m2, in particular 1 to 3 g/m2, from the viewpoints of processability, coating adhesion and the like.
The post-treating agent of the invention is applied to the surface of a phosphate undercoat formed on a metal surface, and dried to form a post-treatment coat.
When applying the post-treating agent of the invention to the surface of the phosphate undercoat on a metal substrate surface, the post-treating agent is usually adjusted to a viscosity of about 5 to 100 mPa≅s with water according to the amount to be applied, and then applied by a process such as roll coating, spray coating, brush coating, flow coating or dip coating. After application, the excess agent may be removed by squeezing. The amount of the post-treating agent to be applied is, as dry film weight, preferably 0.05 to 3.0 g/m2, in particular 0.1 to 2 g/m2, from the viewpoints of corrosion resistance, processability, coating adhesion and the like. The drying conditions are not limited as long as the applied post-treating agent is sufficiently dried. For continuously drying the post-treating agent applied to a phosphate undercoat formed on a metal sheet substrate, the agent is preferably dried in an atmosphere at 100 to 250xc2x0 C. for about 10 to 100 seconds (the maximum temperature reached by the substrate is preferably about 80 to 150xc2x0 C.).
The substrate with the phosphate undercoat which has been post-treated with the post-treating agent of the invention is excellent in corrosion resistance, processability, coating adhesion and like properties, and thus can be used as a rust resistant steel sheet without further treatment.
Optionally, an upper coat may be formed on the post-treated phosphate undercoat. The composition for forming the upper coat can be selected from various coating compositions according to the intended purpose. Examples of coating compositions for forming the upper coat include lubricant film-forming compositions, highly corrosion resistant film-forming compositions, primer compositions and colored topcoat compositions. It is also possible to apply and dry a lubricant film-forming composition, a highly corrosion resistant film-forming composition or a primer composition, and then further apply a colored topcoat composition on the resulting coat.
Undercoating Material for Aluminum or Aluminum Alloy as Metal Substrate
The coating composition of the invention, when used as an undercoating material, is preferable as an undercoating material for aluminum or aluminum alloy substrates.
The undercoating material of the invention is applied to a surface of an aluminum or aluminum alloy substrate and dried to obtain a coated substrate.
The aluminum or aluminum alloy substrate to be coated with the undercoating material of the invention is not limited, but is typically a heat exchanger fin. The heat exchanger fin to be used as the substrate may be known one, which may be a separate member before assembly into a heat exchanger, or a member assembled into a heat exchanger.
The undercoating material can be applied to a surface of an aluminum or aluminum alloy substrate (e.g., a heat exchanger fin), by a known process such as dip coating, shower coating, spray coating, roll coating or electrocoating. The undercoating material is preferably dried for about 2 seconds to 30 minutes under such conditions that the substrate reaches a maximum temperature of about 60 to 250xc2x0 C.
Preferably, the dry film of the undercoating material is usually 0.001 to 10 xcexcm thick, in particular 0.1 to 3 xcexcm thick. A dry film of the undercoating material with a thickness less than 0.001 xcexcm is inferior in corrosion resistance, water resistance and like properties, whereas a dry film of the undercoating material with a thickness greater than 10 xcexcm develops cracks and is reduced in hydrophilicity, processability and the like.
The aluminum or aluminum alloy substrate coated with the undercoating material of the invention can be used as a rust resistant coated substrate without further treatment, since the coat formed on the substrate is excellent in corrosion resistance, hydrophilicity, coating adhesion, processability and like properties. The coated substrate may be irradiated with light to further improve the hydrophilicity.
When the aluminum or aluminum alloy substrate coated with the undercoating material is used as a heat exchanger fin, it is preferable that a hydrophilizing coat is further formed on the coat of the undercoating material.
The hydrophilizing coat has a hydrophilic surface, sufficient strength, high water resistance and good adhesion to undercoats. The hydrophilizing coat can be preferably formed by applying and drying a hydrophilizing composition.
The hydrophilizing composition preferably contains a hydrophilic film-forming binder. Preferred hydrophilic film-forming binders include, for example, (1) an organic resin binder mainly comprising a hydrophilic organic resin and optionally containing a crosslinking agent, (2) an organic resin/colloidal silica binder mainly comprising a hydrophilic organic resin and colloidal silica and optionally containing a crosslinking agent, and (3) a water glass binder made of a mixture of alkali silicate and an anionic or nonionic aqueous organic resin as main components. Among these binders, the organic resin binder (1) and the organic resin/colloidal silica binder (2) are particularly preferred.
The hydrophilic organic resin in the organic resin binder (1) is, for example, a resin containing a hydroxyl, carboxyl or amino group or groups and capable of being solubilized or dispersed in water, in itself or as neutralized with an acid or base selected depending on the functional group or groups.
Specific examples of the hydrophilic organic resin include polyvinyl alcohols, modified polyvinyl alcohols (e.g., copolymers of a polyvinyl alcohol with acrylamide, unsaturated carboxylic acid, a sulfonic acid monomer, a cationic monomer, an unsaturated silane monomer or the like), polyacrylic acids, polyethylene glycols, carboxyl-containing acrylic resins, carboxyl-containing polyester resins, adducts of epoxy resins and amines, copolymer ionomers of ethylene and acrylic acid, and like synthesized hydrophilic resins; starch, cellulose, alginic acid and like natural polysaccharides; and oxidized starch, dextrin, esters of alginic acid and propylene glycol, carboxymethyl starch, carboxymethyl cellulose, hydroxymethyl starch, hydroxymethyl cellulose, hydroxyethyl cellulose and like derivatives of natural polysaccharides.
Examples of the optional crosslinking agent in the organic resin binder (1) include melamine resins such as methyl etherified melamine resins, butyl etherified melamine resins and methyl/butyl etherified melamine resin; urea resins such as methyl etherified urea resins; methyl etherified benzoguanamine resins; polyepoxy compounds such as di- or polyglycidyl ethers of polyphenols or aliphatic polyhydric alcohols, and amine-modified epoxy resins; blocked polyisocyanate compounds such as blocked products of triisocyanurated hexamethylene diisocyanate; and metal chelate compounds of metal elements such as titanium, zirconium and aluminum. The metal chelate compounds preferably have at least two metal alkoxide bonds per molecule. It is generally preferable that the crosslinking agent is soluble or dispersible in water.
The hydrophilic organic resin in the organic resin/colloidal silica binder (2) may be any of the hydrophilic organic resins listed above for use in the organic resin binder (1). The colloidal silica in the binder (2) is a silica sol or a particulate silica, and has a particle size of usually about 5 nm to 10 xcexcm, preferably 5 nm to 1 xcexcm. Usually, colloidal silica provided in the form of an aqueous dispersion can be used as it is, or alternatively, a particulate silica can be used as dispersed in water. The organic resin/colloidal silica binder (2) may be a mere mixture of the organic resin and colloidal silica, or a composite of the organic resin and colloidal silica formed by reaction in the presence of alkoxy silane. The crosslinking agent optionally used in the binder (2) may be any of the crosslinking agents listed above for use in the binder (1).
The aqueous organic resin in the water glass binder (3) may be any of the anionic or nonionic organic resins listed above as hydrophilic organic resins for use in the organic resin binder (1).
The hydrophilic film-forming binders are capable of being dissolved or dispersed in aqueous media. xe2x80x9cAqueous mediaxe2x80x9d means water or mixtures of water as a main ingredient with an organic solvent.
The hydrophilizing composition may consist solely of an aqueous solution or dispersion of a hydrophilic film-forming binder, but may further contain the following ingredients as required: surfactants to improve the hydrophilicity of the coat; particulate hydrophilic polymers (having an average particle size of usually 0.03 to 1 xcexcm, preferably 0.05 to 0.6 xcexcm) to improve the hydrophilicity; 2-(4-thiazolyl)benzimidazole, bis(dimethylthiocarbamoyl)disulfide, zeolite(aluminosilicate) and like antibacterial agents; tannic acid, phytic acid, benzotriazole and like rust preventive agents; oxysalts of metals such as molybdenum, vanadium, zinc, nickel, cobalt, copper and iron; coloring pigments, extender pigments, rust preventive pigments and like pigments; etc.
The hydrophilizing composition is applied to the coat of the undercoating material formed on the surface of an aluminum or aluminum alloy fin (which may have been assembled into a heat exchanger) by a known process, such as dip coating, shower coating, spray coating, roll coating or electrocoating, and dried to obtain a hydrophilized fin for a heat exchanger. The conditions for drying the hydrophilizing composition can be selected according to the type of binder, thickness of the hydrophilizing coat and other factors. The composition is preferably dried for about 5 seconds to 30 minutes under such conditions that the substrate reaches a maximum temperature of about 80 to 250xc2x0 C. The hydrophilizing coat may be cured during drying.
The thickness of the hydrophilizing coat formed from the hydrophilizing composition is not limited, but is preferably 0.3 to 5 xcexcm, in particular 0.5 to 3 xcexcm.
In this manner, a hydrophilized heat exchanger fin made of aluminum or aluminum alloy is obtained.
The following Production Examples, Examples and Comparative Examples are provided to illustrate the invention in further detail, and are not to limit the scope of the claims of the invention. In the following examples, parts and percentages are all by weight.